This application is a 371 of PCT/EP00/11580, filed Nov. 21, 2000.
The present invention relates to an improved process for the isomerization of precursor allyl alcohols to product allyl alcohols in both directions of the equilibrium in the presence of tungsten oxo(VI) complexes comprising additional nitrogen-containing ligands.
Allyl alcohols are important intermediates in industrial organic chemistry. Tertiary allyl alcohols in particular are used, for example, as intermediates in the preparation of fragrances and also as additives in soaps or detergents.
It is known that allyl alcohols isomerize under acidic catalysis. This isomerization corresponds to a 1,3-migration of the hydroxyl group and a corresponding shift of the double bond, as shown in the equation below with the formulae I and II: 
in which R1 to R5 are hydrogen or hydrocarbon radicals.
The process is particularly suitable for the preparation of tertiary product allyl alcohols, such as 2-linalool, by isomerization of primary or secondary allyl alcohols, such as geraniol and nerol.
Geraniol (2-trans-3,7-dimethyl-2,6-octadien-8-ol), nerol (2-cis-3,7-dimethyl-2,6-octadien-8-ol) and 2-linalool (3,7-dimethyl-1,6-octadien-3-ol) are important compounds in the fragrance industry. They are used either directly as fragrances, or are converted into higher molecular weight fragrances by reaction with other compounds. These terpene alcohols are also important as C10 building blocks in the synthesis of vitamins, such as vitamin E.
In the past, preference has been given in the literature to the description of processes for the isomerization of linalool to geraniol. Since the isomerizations are equilibrium reactions, the processes developed can, in principle, also be used for the reverse reaction of the isomerization of geraniol or nerol to linalool.
Initially, the isomerization reactions of allyl alcohols were carried out using acids as catalysts. However, these processes were of only limited importance since during them secondary reactions, such as, for example, dehydrations or cyclizations, predominate.
Later, the catalytic rearrangement of substituted allyl alcohols using molybdenum, vanadium and tungsten catalysts was investigated (cf. P. Chabardes et al in Tetrahedron 33 (1977), pages 1775-1783.).
Whereas the molybdenum compound described in GB 1 256 184 as isomerization catalyst produced unsatisfactory reaction results, using tungsten oxo(VI) alkoxide catalysts of the formula WO(OR)4 in the presence of a nitrogen base as additional ligand, relatively high selectivities coupled with simultaneously higher activities, compared with the analogous vanadium oxo(V) alkoxide catalysts of the formula VO(OR)3, were possible. Further advantages of the tungsten catalysts are that they can be readily separated off from the reaction mixture (cf. T. Hosogai et al. in Chemistry Letters 1982, pages 357-360) and that they have only low toxicity compared with the vanadium catalyst.
Furthermore, DE 25 16 698 discloses the preparation of novel catalysts based on tungsten, and to the use thereof for the catalytic rearrangement of tertiary to primary allyl alcohols. In this process, the catalysts described are tungsten oxo(VI) complexes comprising alkoxy radicals and/or trialkylsilyl radicals bonded via oxygen, which, to improve the selectivity, additionally comprise ligands bonded coordinately to the tungsten, which comprise an element chosen from the elements N, P, As and Bi, in particular ligands chosen from the class of primary, secondary and tertiary monoamines, of polyamines, of Schiff""s bases, of imines, nitriles and isonitriles. Ligands which are cited therein as particularly suitable are primary monoamines, such as methylamine, ethylamine, propylamine, xcex2-ethoxyethylamine, butylamine, cyclohexylamine, alinine and naphthylamine; secondary monoamines, such as dimethylamine, diethylamine, dibutylamine, dicyclohexylamine, methylaniline, methylcyclohexylamine, piperidine, morpholine and pyrrolidine; tertiary monoamines, such as trimethylamine, triethylamine, ethyldibutylamine, tricyclohexylamine, dimethylaniline, pyridine, picoline, quinoline, isoquinoline, N-methylpyrrolidine and N-methylmorpholine; ethylenediamine, pyrazine, piperazine, pyrimidine, triethylenediamine, 1,8-diazabicyclo[5.4.0]undec-7-ene, polyethyleneimines, and ion exchanger resins having a large number of amino groups within the molecule, in particular pyridine, triethylamine, cyclohexylamine, diethylamine and tricyclohexylphosphine. Aminoalcohols are not mentioned therein. The selectivities for primary alcohols, such as nerol and geraniol, achieved with this process are extremely good, although the conversions achieved are still not ideal.
Our own investigations into the isomerization of geraniol using a 0.05 mol % strength solution of tungsten oxo(VI) tetrakis geranylate analogously to the process described in DE 25 16 698 have shown that the rearrangement to linalool proceeds in a significantly more selective manner in the presence of a nitrogen base at 200xc2x0 C. (reaction time about 1 hour) than without co-use of a nitrogen base (see Comparatives Examples 1 to 5).
The nitrogen bases used here were diethylamine, pyridine, imidazole and poly(4-vinylpyridine).
Disadvantages of these experiments were the comparatively low conversions coupled with simultaneously high temperatures of more than 150xc2x0 C., which accelerated the formation of by-products.
It is an object of the present invention to improve the process for the isomerization of allyl alcohols in the presence of tungsten oxo(VI) complexes comprising additional nitrogen-containing ligands at temperatures of from 50 to 300xc2x0 C. such that, even in the case of the isomerization of primary or secondary allyl alcohols, such as geraniol and nerol, to tertiary allyl alcohols, such as linalool, relatively high conversions of the precursor allyl alcohol used are achieved, as a result of which the rate of the establishment of an equilibrium is accelerated, and the space-time yields increase.
Surprisingly, we have found that the process for the isomerization of precursor allyl alcohols to product allyl alcohols in the presence of homogeneously dissolved tungsten oxo(VI) complexes comprising additional nitrogen-containing ligands gives, coupled with selectivities which remain the same, higher activities for the isomerization of geraniol and nerol to linalool than corresponding isomerizations using tungsten oxo(VI) alkoxide catalysts in the presence of the known nitrogen bases alone if aminoalcohols are used as additional nitrogen-containing ligands.
A higher activity during the allyl rearrangement also results when the amino alcohols are added only subsequently to tungsten oxo(VI) alkoxide catalysts of the formula WO(OR)4, to which nitrogen bases have already been added as additional ligands.
Accordingly, the invention provides a process for the isomerization of precursor allyl alcohols to product allyl alcohols in the presence of either preprepared or in-situ-generated tungsten oxo(VI) complexes comprising additional nitrogen-containing ligands at temperatures of from 50 to 300xc2x0 C., wherein, in the catalyst, as well as the nitrogen bases known as additional nitrogen-containing ligands, or instead of these nitrogen bases, aminoalcohols are present as additional nitrogen-containing ligands.
Examples of allyl alcohols which can be advantageously isomerized using the process according to the invention and which may be mentioned are:
2-Methyl-3-buten-2-ol, prenol (3-methyl-2-buten-1-ol), linalool, nerol and geraniol, and farnesol (3,7,11-trimethyldodeca-2,6,10-trien-1-ol) and nerolidol (3,7,11-trimethyldodeca-1,6,10-trien-3-ol), in particular linalool, nerol and geraniol.
The aminoalcohols which can be used in the process according to the invention are aminoalcohols of the formula III, 
in which R8 and R9 are identical or different and are hydrogen, C1- to C4-alkyl, xe2x80x94CH2xe2x80x94C6H5OH; xe2x80x94C2H5OH or xe2x80x94C3H7OH,
R10 and R11 are identical or different and are H, C1- to C4-alkyl, or together form a saturated or unsaturated divalent radical optionally substituted by C1- to C4-alkyl groups,
R12 and R13 are identical or different and are hydrogen or a C1- to C4-alkyl radical,
and n is an integer from 0 to 5.
The process according to the invention proceeds particularly advantageously if the aminoalcohols used are triethanolamine, diethanolamine, monoethanolamine, tripropanolamine, dipropanolamine, 3-amino-1-propanol, 1-amino-2-propanol, 2-amino-1-propanol, butyldiethanolamine, methyldiisopropanolamine, N-(2-hydroxybenzyl)amine or N,Nxe2x80x2-bis(2-hydroxybenzyl)-1,2-diaminoethane.
For the preparation of the isomerization catalysts required for the process according to the invention, the tungsten oxo(VI) complexes used are generally the tungsten oxo(VI) complexes or tungsten oxo(VI) compounds described in DE 25 16 698 of the formulae
[WO(OR1)(OR2)(OR3)(OR4)]nL
or
WO(OR1)(OR2)(OR3)(OR4),
in which R1 to R4 are hydrocarbon groups, substituted hydrocarbon groups or groups of the formula 
where R5, R6 and R7 are each hydrocarbon groups which may optionally be substituted, where R1, R2, R3 and R4 may be identical or different and, if two or more of the groups R1, R2, R3 and R4 are hydrocarbons, irrespective of whether they are substituted or unsubstituted, two groups of this type can form a cyclic structure together with the adjacent tungsten and oxygen atoms in the formula, preferably compounds in which R1, R2, R3, R4, R5, R6 and R7 are hydrocarbon groups chosen from the following groups: alkyl groups having 1 to 20 carbon atoms, alkenyl groups having 2 to 20 carbon atoms, cycloalkyl groups having 3 to 10 carbon atoms, cycloalkenyl groups having 3 to 10 carbon atoms, aryl groups having 6 to 10 carbon atoms, aralkyl groups having 7 to 20 carbon atoms, and alkylaryl groups having 7 to 20 carbon atoms, in particular compounds in which R1 and R2 are as defined above, R3 and R4 are hydrogen or a methyl group, or together form a saturated or unsaturated divalent radical, and R5, R6 and R7 are hydrogen, L is a monovalent or polyvalent ligand which comprises nitrogen and is coordinately bonded to the tungsten atom in the formula via this nitrogen, and n is a value between 1 and the number of tungsten atoms which can be coordinately bonded to the ligand L.
The tungsten oxo(VI) complexes can be prepared, for example, in accordance with the processes described in DE 25 16 698 from tungsten trioxide with the hydroxyl compounds which contain groups corresponding to R1, R2, R3 and R4 in the presence of the ligand compound; or by reacting a tungsten oxytetrahalide with an alkali metal alkoxide of a hydroxyl compound containing groups corresponding to R1, R2, R3 and R4 in the presence of the ligand compound.
The tungsten oxo(VI) complex can, as described in DE 25 16 698, be prepared before the actual reaction, or else also be prepared directly in the reaction mixture.
For example, it is possible to add sodium tungstate dihydrate as solid to the precursor allyl alcohol, in this case geraniol. By heating the mixture to the temperature desired for the isomerization, the isomerization catalyst forms in the reaction mixture. The water eliminated during the catalyst formation is removed together with the isomerization product, in this case linalool, by reactive distillation.
The disclosure of DE 25 16 698 is therefore incorporated into this patent application by citation.
Generally, the tungsten oxo(VI) complexes are used dissolved in the precursor allyl alcohol in a concentration of from 0.001 to about 5% by weight.
The aminoalcohols are generally also advantageously used dissolved in the precursor allyl alcohol. They are generally used in an amount of from 1 mol % to 1000 mol %, preferably from 33 mol % to 500 mol %, based on the tungsten compound.
The set amount of the aminoalcohol ligand and, in particular, the amount of ligand compared with the amount of tungsten oxo(VI) complex used has an effect on the rate and the selectivity of the reaction.
A small amount of aminoalcohol ligand relative to the amount of tungsten effects a high rate of reaction, but also a somewhat lower selectivity, whereas a large amount of ligand produces the product with high selectivity but, under certain circumstances, a lower space-time yield.
The aminoalcohols are preferably used in an amount of from 100 mol % to 400 mol %, based on the total number of starting hydroxyl groups in the aminoalcohol and on the amount of tungsten, in particular in an amount of from 300 mol % to 400 mol %, based on the total number of starting hydroxyl groups in the aminoalcohol and on the amount of tungsten.
The absolute concentrations of ligand and tungsten complex in the reaction mixture are unimportant in the process according to the invention and can, for example, be increased such that the rate of the establishment of an equilibrium is increased in a desired manner.
An increase in the rate of the establishment of an equilibrium can also be achieved in the process according to the invention by removing water of reaction which has formed from the mixture, for example by passing over a stream of inert gas, adding known water-removing agents, or stripping with the stream of gas during the distillation.
In the tungsten oxo(VI) complexes, all or only some of the alcohol radicals on the tungsten atom which act as ligands, and all or only some of the nitrogen bases which act as additional ligands, can be replaced by the aminoalcohols added as additional ligands.
The process according to the invention is generally carried out at temperatures of from 50 to 300xc2x0 C., preferably at from 150 to 250xc2x0 C.
It can be carried out with and without co-use of a solvent, discontinuously, or else continuously.
Solvents which have proven particularly advantageous are, for example, the high-boiling secondary compounds of the allyl alcohol isomerization.
The process according to the invention can be advantageously carried out if the precursor allyl alcohols in the reaction mixture are present in a concentration of from about 10% by weight to 100% by weight.
The reaction mixture is worked up in a manner known per se.
A particularly advantageous process according to the invention is one where the precursor allyl alcohols used are geraniol and nerol, and the product allyl alcohol prepared is 2-linalool.
In this process, for work-up, 2-linalool is separated off from the product mixture as the lower-boiling component by distillation. In general, precursor allyl alcohols and secondary compounds will be present in the product allyl alcohol. The product allyl alcohol can be purified by distillation by known methods.
The isomerization is an equilibrium reaction and the position of the equilibrium depends on the thermodynamic properties of the precursor and product allyl alcohols, and on the reaction conditions. The discontinuous or continuous removal of linalool, the lowest-boiling allyl alcohol in the mixture, from the reaction batch permits, even in the case of an unfavorable establishment of an equilibrium as a result of the shift of the equilibrium, a favorable space-time yield, the reboiler of the distillation column serving as reaction chamber.